In recent years, the introduction of safety equipment is being actively promoted in the automotive industry. Among such equipment, airbag systems which rapidly expand an airbag with gas or the like between a passenger and a steering wheel, an instrument panel, or the like at the time of a collision before the passenger impacts the steering wheel, etc. so as to absorb the kinetic energy of the passenger and decrease his or her injury have been developed and installed in most automobiles.
Airbag systems using explosive chemicals to expand an airbag were generally employed in the past. However, in order to permit environmental recycling, airbag systems in which an airbag is expanded using a confined high pressure gas have been developed and are increasingly being used.
In the airbag system using a confined high pressure gas, a gas for expansion such as an inert gas (e.g., argon) which is blown into an airbag at the time of a collision is confined in a pressure accumulating vessel (accumulator) and always maintained therein at a high pressure. The gas is blown out all at once from the accumulator into an airbag at the time of a collision. The accumulator is generally fabricated by welding a lid to each end of a steel tube which has been cut to an appropriate length.
An accumulator for airbags (referred to herein as airbag accumulator or simply as accumulator) is always filled with a high pressure gas at around 300 kgf/cm2, for example, and it must withstand such a high pressure for a long period. In addition, when the gas is blown out therefrom, the accumulator is subjected to stress at a high strain rate in an extremely short period of time, and it must withstand such a stress, too. In order to make it possible to reduce the size and weight of an airbag system, which leads to saving of milage of an automobile, an airbag accumulator is desired to have an increased gas pressure and a decreased wall thickness.
Accordingly, a seamless steel tube, which is generally more reliable than a welded steel tube at a high pressure, is used in the fabrication of an airbag accumulator. In contrast to a simple structure such as conventional pressure cylinders or line pipe, a steel tube for an airbag accumulator is desired to have a high tensile strength on the order of at least 850 MPa in order to sufficiently withstand the filled gas pressure and excellent low temperature burst resistance (or toughness) as indicated by a ductile fracture occurring in a bursting test at a temperature of −20° C. or below in view of the possibility of use at low temperatures, in addition to a high level of dimensional accuracy, workability, and weldability.
Seamless steel tubes suitable for use in an airbag accumulator and methods for their manufacture are disclosed in Patent Documents 1 to 4 cited below, for example.
In the methods proposed in these patent documents, a seamless steel tube having desired high strength and excellent burst resistance is manufactured by a process including quenching and tempering. However, heat treatment for quenching and tempering has the problem that it makes the manufacturing process complicated thereby decreasing productivity and increasing manufacturing costs. Accordingly, there is a demand for a method of manufacturing a seamless steel tube which can satisfy desired properties using heat treatment which can be performed easily.
Patent Document 5 discloses a method of manufacturing a seamless steel tube for an airbag accumulator in which quenching and tempering are not carried out as heat treatment. In this patent document, it is described that a high strength, high toughness steel tube having high dimensional accuracy and good workability and weldability can be manufactured by subjecting a steel tube as formed to normalizing at 850-1000° C. followed by cold working to obtain prescribed dimensions and optionally further followed by stress relief annealing, normalizing, or quenching and tempering. However, the technology disclosed in Patent Document 5 aims to manufacture a seamless steel tube having a tensile strength on the order of 590 MPa, and the values of tensile strength of the steel tubes which are obtained in the examples set forth therein is at most 814 MPa, which is insufficient to meet the demands for an increase in the pressure of filling gas and a decrease in weight due to a decrease in wall thickness in recent airbag accumulators.
Similarly, Patent Document 6 discloses a seamless steel tube for an airbag which is manufactured by cold working without heat treatment or with heat treatment in the form of annealing, normalizing, or quenching and tempering, with the object of attaining a tensile strength on the order of 590 MPa or higher. That document only discloses the type of heat treatment after cold working with no limitations on the conditions of the heat treatment, from which it is apparent that the object is to be achieved by means of the steel composition alone.
A method of manufacturing a seamless steel tube for an airbag having high strength, high toughness, and high workability in which heat treatment is carried out by normalizing instead of quenching and tempering is proposed in Patent Document 4.
In this method, a steel material having a composition comprising C: 0.01-0.10%, Si: at most 0.5%, Mn: 0.10-2.00%, Cr: greater than 1.0% up to 2.0%, Mo: at most 0.5%, and optionally at least one of Cu: at most 1.0%, Ni: at most 1.0%, Nb: at most 0.10%, V: at most 0.10%, Ti: at most 0.10%, and B: at most 0.005% is used to form a seamless steel tube, and the tube is then subjected to normalizing by heating at a temperature in the range of 850-1000° C. followed by air cooling and then to cold drawing to obtain prescribed dimensions. However, there are few examples relating to the conditions of normalizing. In addition, since the method is premised on a Cr content exceeding 1.0%, the alloy costs are relatively high, and the low temperature toughness is questionable.
In Patent Document 4, low temperature toughness is evaluated by a drop weight test. A drop weight test is also utilized in Patent Document 6 and other publications as a simple method for evaluating low temperature toughness. However, in Patent Document 6, seamless steel tubes which have undergone heat treatment such as annealing and those which are as cold worked were evaluated as having the same low temperature toughness in a drop weight test. In view of this, it is unconvincing that a drop weight test, which is merely a simple evaluation method, can properly evaluate the strict performance requirement desired for present-day airbag accumulators.
As suggested in the above-described patent document, cold working such as cold drawing is indispensable in the manufacture of a seamless steel tube for an airbag accumulator in order to increase the dimensional accuracy of outer diameter and wall thickness. As described in paragraphs 0003-0004 of Patent Document 7, an airbag accumulator is a part for which good dimensional accuracy of outer diameter is required for its assembling, but an increase of the wall thickness of the steel tube of an accumulator in order to increase its strength cannot be employed since it is necessary to avoid an increase in the weight of an automobile. In addition, an airbag is now installed not only for a driver seat, but for passenger seat and even for rear seats, and in order to install a plurality of airbags in one automobile, there is a growing demand for reduction of costs of accumulators.
Patent Document 1: JP H08-325641 A1
Patent Document 2: JP H10-140250 A1
Patent Document 3: JP 2002-294339 A1
Patent Document 4: JP 2004-27303 A1
Patent Document 5: JP H10-140249 A1
Patent Document 6: JP H10-140283 A1
Patent Document 7: JP H11-199929 A1